In accordance with the present invention, a mold, method and improvement process are provided for creating a large, trapezoidally-shaped opening in a thermoformed container. Thermoformed containers have gained wide acceptance for use as food containers for single use. These disposable containers serve as storage, transport and serving means for a wide range of various comestibles. Their use ranges widely from so called "take-out" establishments to "fast food" retail restaurants. The containers are constructed from a thermoplastic material, such as foamed polystyrene or the like and generally comprise a tray member adapted to receive the comestibles and a hingedly connected cover member which is held closed by some type of latching mechanism. The efficacy of the latching mechanism has a great influence on the performance of the container and its acceptance by customers. Latching mechanisms that hold the cover closed very securely tend to be difficult for customers to open for access to the comestibles therein. Conversely, latching mechanisms that are easily opened are prone to accidental opening while being transported leading to spillage of the contents and extremely negative customer reactions to the container. Therefore latch design and particularly the size of the opening used for a latch have required a compromise between providing a secure closure and a easy to open closure.
The ability to create latching mechanism openings during the thermoforming process is well known in the art of making thermoformed containers. In fact, in-mold perforation of latching mechanism openings has largely replaced the use of post forming punching operations.
U.S. Pat. No. 4,909,721 to Warburton describes a male and female mold parts combination for forming a latch opening in a thermoplastic container. The mold features a female mold key and a male mold key, a cutting edge adjacent to a planar face of one key and a vertical surface having a beveled edge surface on the other key. As the mold halves come together during the thermoforming process the cutting edge of one key engages and rides off the beveled edge surface of the other key and then slides across the vertical surface piercing and enlarging a latch opening in the thermoplastic material. The two keys that create the opening assume a contacting parallel relationship during their engagement.
U.S. Pat. No. 4,446,088 to Daines, the contents of which are incorporated herein in its entirety, describes a process and a mold for the production of a thermoformed carton. The patent discloses a male cutting key having a flat cutting surface parallel to the closing motion between a male mold member and a female mold member. It further discloses a female key having a flat cutting surface which is also parallel to the closing motion between the male and female mold members. A biasing means urges the flat surfaces of the male and female keys against each other to create a latch opening as the male and female mold members close on a preheated thermoplastic sheet. In this mold and process the male key is movably mounted in the male mold so as to travel a path perpendicular to closing motion.
Vertical shearing is also utilized in U.S. Pat. No. 4,155,692 to Kermoian which describes a mold for forming in one operation a container with a latch opening and a reinforces area adjacent thereto. Shearing and removal of the sheared sheet is accomplished by the vertical movement of a shear bar past a vertically aligned die face during compression of the male and female mold members. The shear bar entrains a portion of the thermoplastic sheet being formed and moves it downwardly into a cavity of where it is compressed into an area adjacent to the latch opening.
A spring-oriented rotary shear key is used in the mold described in U.S. Pat. No. 5,162,123 to Grimes. Here again two shear keys, an upper key and a lower key, are used to create a latch opening during the vertical closing of two mold members. The lower key is pivotally mounted on a pivot pin extending along an axis normal to the path of travel of the of the mold members.
However, all of the teachings of those patents require that the cutting action be accomplished via the sliding, parallel engagement of two vertically aligned shear keys. All shear key movement is either parallel or perpendicular to the motion of closing of the mold members. While each of these inventions will cut an opening in a thermoformed container, they have common disadvantages and limitations. A particular disadvantage is that the edges of the resulting latch openings tend to cause some amount of tearing as the opening is formed. Ragged edges on the latch opening affect the operability, and customer acceptance of the finished product. A significant limitation on the vertical/parallel approach is the size of the opening created. None of the prior art described herein above produces an opening that is simultaneously very wide and very tall. The maximum height of the opening created by each of the prior art patents discussed above is about 9.53 mm (3/8 in). Widths far in excess of that dimension have been obtained, however, the openings have always been limited to the 9.53 mm height dimension.
The limitations on latch opening height and width have indirectly limited the size of the protrusion or button that extends through the latch opening. These protrusions are typically pushed through the latch opening by the users thumb as a container is opened. Containers having buttons that are very much smaller than the area of a user's thumb can be difficult to open. There is therefore a need in the art for an easy to open latching mechanism that provides a secure container closure and has a large protrusion or button providing ease of customer use.